Generally described, a combined cycle system uses a combination of a gas turbine and a steam turbine to produce electrical power or otherwise drive a load. Specifically, a gas turbine cycle may be operatively combined with a steam turbine cycle by way of a heat recovery steam generator and the like. The heat recovery steam generator is a heat exchanger that allows feed water for the steam generation process to be heated by the hot combustion gases of the gas turbine exhaust. The primary efficiency of the combined cycle system arrangement is the utilization of the otherwise “wasted” heat of the gas turbine engine exhaust. Specifically, the efficiency of the heat recovery steam generator depends on the heat transfer between the gas turbine combustion gases (“the hot side”) and the feed water and the steam (“the cold side”). The design aim is to generate maximum possible useful work from the heat in the gas turbine exhaust.
Although a combined cycle system is efficient, there are numerous types of parasitic losses as well as routine operating losses. For example, one source of wasted energy is the flow of hot, high pressure steam that bypasses the steam turbine during startup, shutdown, and contingency events. Instead of producing useful work, this flow of steam is generally routed directly to the condenser. In fact, this flow of steam is generally attemperated with feed water to drop the temperature below a required condition.
There is thus a desire for an improved combined cycle system with reduced parasitic and operating losses. Preferably, otherwise wasted flows of hot, high pressure steam may provide useful work for supplemental power generation and improved overall efficiency.